AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Study of Vehicle-emitted Nonvolatile Particles in a Traffic Tunnel Using a Thermodenuder: Measurements and Modeling
XIANG LI, Yutong Guo, Timothy Dallmann, Satbir Singh, Albert A. Presto, Carnegie Mellon University
Abstract Number: 548 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract Traffic-emitted particulate matter (PM) is an important contributor to the atmospheric PM in the urban environment, which could potentially become cloud condensation nuclei (CCN) and therefore contribute to climate change. Previous work has shown that it is important to reduce the uncertainty in primary emission rates in order to reduce the uncertainty in predictions of CCN concentrations.
In the present work we have studied the size distributions of vehicle-emitted PM in a traffic tunnel in Pittsburgh, PA. PM with sizes of 7 – 500 nm were measured by a pair of scanning mobility particle sizers. A thermodenuder (TD) was held at a high temperature (250 C) and a short residence time (3 seconds) to quantify the size distribution of nonvolatile PM. After evaporating under high temperature, the nonvolatile PM appeared at two modes downstream of the TD – one consisting of EC with a peak at 80-100nm, and another consisting of particles smaller than 10 nm. The chemical composition of the smaller nonvolatile mode is uncertain.
In order to understand the characteristics of the smaller nonvolatile mode, TD experiments are simulated using a Computational Fluid Dynamics (CFD) model and the TwO-Moment Aerosol Sectional (TOMAS) microphysics model. First, we simulate the condensation of semivolatile organics onto the EC particles traveling from vehicle tailpipe to the sampling inlet to predict the size-dependent OA-to-EC ratio in the particle phase. Then, we allow the aerosol particles to evaporate inside the TD. Preliminary modeling results suggest that particles with sizes of 20 - 70 nm have very high OA-to-EC ratio ranging from 6 to 30 and that the smaller non-volatile mode could potentially come from the shrinkage of larger particles when exposed to such high temperatures inside the TD.